WOODEN BRIDGES, are platforms constructed of car pentry, for crossing streams, rivers, roads, Sze. These bridges may be looked upon as the origin of all such constructions, whether of stone, wood, or iron. In early times it may fitirly he supposed, that trees, or planks made from them, weold be thrown front one bank of the stream to the other, so as to serve for a footway, by which a passage might be effected. This might have been suggested in the first instance by accident, either by a tree lidling in that position, or else by its growing across a stream, as sometimes happens with willows, &c. A plank placed from one bank of a stream to another, is the simplest form of a timber bridge; it is at the same time the most perfect ; and the principle on which it is suspended, or kept in its proper position, is worthy of con sideration, for we may learn how to construct the best and most advantageous kind of bridge, suitable for immense spans, from this unprutending, and apparently unpremedita ted contrivance.
When a strong plank is thus laid upon two supports, that part of it which lies midway between them, has to sustain its own weight, together with the transit load which it has to bear, such as that of anything crossing over it, by the cohe sion between its particles, tor as that part of the plank has nothing to rest upon, it is therefore clear, that it has a ten dency to break somewhere between the points of support when the strain becomes very great upon it ; but owing to this cohesion of the particles, which attracts them to one another, such a plank cannot snap asunder, because the fibres of the timber of which it is farmed, are so interwoven, that one particle or atom of the material will not readily he separated from its fidlow, as long as such material remains in a sound state. This being the case, the effect of the weight upon the beam will cause it to bend, or what is technically termed, to sag, but in order to this, it will be necessary that the fibres on the under side of the beam should first be drawn out, or lengthened, that is, subjected to a tensile strain, while on the contrary, those on the tipper surface will he forced to contract, or become shorter. in other words, they will be pressed upon longitudinally, and thus be subjected to a compressive fbrce. It is also evident, that there must be some intermediate plane between the tipper and lower surfilees of the where the two opposite con tending forces will meet, in which, of ceurse, neither will preponderate ; this, which is denominated the neutral plane, is situated lower or higher, according to the depth of the beam, the homogeneity and cohesion of the material of which it is composed, and a variety of other considerations. As a
general for !haling the weight with which it v•ould be advisable to load such a beam, we may give the following, to 2 1, d2s 3 / , wnere to is the weight in lbs. with which the beam may be loaded, including that of the beam itself, 1 its breadth in inches, d its depth in inches, 1 its length in inches and s smile known constant, representing the calcu lated strength of the kind of timber, or other material, employed. That larch has been estimated at 2S0, Riga fir at ;3'7G, English oak 400, Mar Forest fir 415, red pine 417, pitch pine 511, Canadian oak SSS, ash (175, teak S20. When the inertia of such a bridge is small, the deflection, or sagging, of the beam is found to depend upon a certain quantity, which varies directly as the square of its length, and inversely as the product of the central statical deflection, (that is, the amount of deflection which would take place if all the load were placed at the centre of the beam.) and with the square of the velocity with which the load passes over the bridge.
In a bridge, therefore, constructed with a single beam, there are two counteracting forces, mutually neutralizing each other, prevent the beam of which it consists from yield ing ; this constitutes what is called the rigidity of the timber, which thus opposes an obstacle to its bending, and which is increased in the direct proportion ofthesquare of the depth and inversely as its length within certain limits, provided it is not loaded to more than a third of its calculated capability, which is taken into consideration in the above formula. This form of bridge merely rests upon its abutments, or piers, pressing vertically upon them. It exerts no force tending to over turn or push against them in an oblique direction. In this consists its superiority over bridges which are constructed solely on the principle of compression, such as arched bridges in general, or bridges formed completely on the principles of tension, as in the case of suspended bridges, in both of which it is necessary to introduce some countervailing weight or pressure, so as to resist the tendency which both those des criptions of bridges have of oversetting their supports. This adds greatly to their complicated nature, and consequently to the difficulty of their construction, involving, as it does, nice calculation, founded upon careful observation and experhnent.